Architecture and management of a geological repository - Andra

7 – The shafts and the drifts7.7.2.1 Design principlesFigure 7.7.2Drift seal diagram (with hydraulic cut-offs of the fractured zone)Swelling clay is adopted as the base material for the seal core given its favourable properties. Its verylow permeability satisfies the objective of not allowing water to circulate in the drifts. Its swelling anddeformation capability when water is present means that it fills the clearances left during theconstruction of the core and provides a good contact with the drift wall; once the clay is swollen, thewater has no possible flow path through the core. Its natural character and chemical compatibility withargillite guarantee a high durability.The core hydro-mechanical properties can be adjusted by varying the method of implementation andthe formulation of the material. The potential swelling pressure of the clay increases with its averagedry density in the engineered structure. This density depends on the compaction rate of the clay, itselfa function of the implementation method. In terms of formulation, adding sand, for example, mayencourage compaction and improve the mechanical properties of the material without significantincrease in permeability.For the clay in the core to develop and maintain a swelling pressure in the presence of water in thelong term, it must be located in a contained volume. Thus the role of the concrete retaining plugs is tolimit the core expanding in volume; to achieve this, they must resist the pressure developed by the claymechanically, particularly during the transient phase of core hydration and swelling. A very long-termchemical alteration of the concrete cannot be excluded, which would reduce its properties; to preventsuch an alteration reducing the core performance, support backfilling will then take over the role of theconcrete retaining plugs.Note that these design principles for the drift seals are similar to the plugs for the C waste disposal cellpresented in Chapter 5.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM310/495